DNA ligase is an enzyme having the activity of linking DNA chains by forming a phosphodiester bond between a 3′-hydroxy group and 5′-phosphoryl group of DNA and is involved in DNA replication and repair of damaged DNA strands in vivo. DNA ligase is also used in a recently developed gene amplification technique known as a ligase chain reaction (LCR). LCR is a technique by which a target gene is amplified or detected through a temperature-cycling reaction using a heat-resistant DNA ligase. For more efficiently carrying out LCR, heat-resistant ligases with higher activities have been searched and commercially supplied.
DNA ligases derived from hyperthermophilic archaeon and having excellent thermal stability have been recently found (National Institute of Advanced Industrial Science and Technology (AIST) of Japan, On-line Press Release (2003): http://www.aist.go.jp/aist_j/press_release/pr2003/pr20030910/pr20030910.html; “Development of Extremely Heat-resistant DNA Ligase for Genetic Diagnosis”; corresponding to JP-A NO. 2004-248636 and US-A No. 20040259123). These heat-resistant DNA ligases excel in thermostability but have a disadvantage of poor reactivity, because they have a very poor binding ability to DNA. In contrast, phage-derived DNA ligases are known as DNA ligases having high binding ability to DNA (hereinafter briefly referred to as “DNA binding ability”). These DNA ligases are, however, poor in thermal resistance and thereby not suitable for LCR. Thus, no DNA ligase that has high thermal resistance and high DNA binding ability and makes it possible to carry out LCR at a sufficient turnover has yet been found.